The simple fact is that bike shop mechanics are like all other specialists; auto mechanics, plumbers, appliance repairmen, even dentists and doctors, in that they range from highly skilled to just just slightly above inept.

You have two choices, either learn how and be willing to do all your work yourself, or learn how to find decent a decent mechanic. Do it the same way you'd approach finding a decent dentist; get referrals. Find the local bike club, or go to where local rides start and ask about who does decent work locally.

Whatever you do, don't just assume that because it says bicycle sales and service over the door, the staff inside are knowledgeable, skilled, caring, or even honest. Or because they know how to work on some bikes they know how to work on all bikes.

A bit of due diligence on your part will pay off with better service and a bike that runs better for a lot less money.